In recent decades, contact lenses have become a preferential alternative to other eyesight correction methods. Due to their increased popularity, it has become mandatory that contact lenses be manufactured on a large scale in order to meet consumer demand. Further, these lenses are required to be precision manufactured with low tolerances in order to provide a suitable and effective corrective lens.
Spin casting has been utilized as a method of producing contact lenses. However, traditional spin casting methods are disadvantageous for several reasons as will be discussed below, and have not readily been employed in the mass-production of contact lenses.
To begin, the polymerization casting of axially symmetrical articles, such as contact lenses, may be performed by using a spin casting process. In this process, a controlled quantity of a polymerizable liquid is placed into an open mold, which is then rotated about its vertical axis at a rotational speed sufficient to produce a centrifugal force that causes a radially outward displacement of the polymerizable liquid. By maintaining a controlled rotation rate, the centrifugal force caused by the rotation will cause the polymerizable liquid to adopt a generally concave shape. Once the polymerizable liquid has attained an equilibrium shape, polymerization of the liquid can be effected by any suitable means, such as heat or exposure to actinic radiation (i.e. ultraviolet light) so as to produce a solid polymeric contact lens.
The open mold used in a spin casting process is typically characterized by an outer cylindrical wall and a mold comprising an exposed concave molding cavity. The shape of the molding cavity will typically define the shape of the front surface of the finished contact lens, and may contain such desired elements as lenticulating curves, toric curves, non-spherical curves and other such features or shapes aimed at interacting with the eye, its optical processes, or eyelids in a predetermined manner.
The shape factor of the posterior or back surface of the lens is determined predominantly by the angular speed of rotation, as well as other factors such as the surface tension of the polymerizable liquid, and the acceleration due to gravity.
The polymerizable liquid utilized in the spin casting process is typically one in which the polymerization reaction can be triggered by an external factor such as heat or actinic radiation (i.e. ultraviolet light), and is therefore most commonly utilized in connection with a system that undergoes a free radical polymerization reaction. Typically these systems will include a monomer, or mixture of monomers based on acrylic or methacrylic acid, along with a free radial polymerization initiator. However, pre-polymerized materials such as solvent-based materials may also be applied in a spin casting system.
To avoid the inhibiting effects of atmospheric oxygen during the polymerization process, the molds and polymerizable liquid are maintained, at least initially, in an inert gas atmosphere of, for example, nitrogen or argon. The use of an external trigger for the polymerization allows for the polymerizable liquid to attain its equilibrium shape under rotation prior to the onset of polymerization, and also to allow time for any oxygen present within the mold or dissolved in the polymerizable liquid to diffuse away from the polymerizable liquid.
During the actual mass production of contact lenses, the individual molds can be arranged in a carousel or in a vertical stack configuration. The carousel arrangement is rather complex and quite large with respect to the size of the molds. It requires that each mold be individually rotated on its own separate vertical axis. It is reported that the carousel arrangement suffers from the disadvantages of requiring excess inert gas to eliminate the inhibiting effect of oxygen (in the ambient environment) present during the polymerization reaction. The use of excess inert gas during the polymerization of the monomeric reactants causes the entrainment of monomer in the form of vapors and the subsequent deposition and/or polymerization of the monomer on the surrounding objects, and, in particular, the equipment utilized by the system. Further information is set forth in Method of Centrifugally Casting Thin Edged Corneal Contact Lenses, U.S. Pat. No. 3,660,545 to Otto Wichterle (filed Oct. 24, 1963) (issued May 2, 1972), the full disclosure of which is incorporated herein by reference in its entirety.
In the vertical stack arrangement a rotatable polymerization tube having an internal, generally circular, cross-sectional geometry is adapted to receive, at one end of the tube, a plurality of generally circular molds which become seated to one another in the tube, each mold containing the polymerizable liquid reactants in their individual mold cavities. The polymerization tube, or rotatable tube, can be manufactured so that its internal diameter generally matches the external diameter of the individual molds so as to provide an interference fit. More preferably, the rotatable tube can contain ridges or similar features so as to facilitate a multiple point contact with the individual molds. This latter arrangement allows for the molds to rotate with the rotatable tube, and also to allow for the passage of inert gas through the rotatable tube and past the individual molds. Suitable prior art designs for the rotatable tube are disclosed in Device and Method for Centrifugally Casting Articles, U.S. Pat. No. 4,517,138 to David L. Rawlings et al. (filed May 2, 1983) (issued May 14, 1985) (hereinafter “'138 patent”).
One typical prior art arrangement for the production of lenses by spin casting is that taught by the '138 patent. In this design, monomer dosed molds are fed, one by one, into the top of a rotatable tube comprising two zones, a conditioning zone and a polymerization zone. Typically the rotatable tube contains a plurality of dosed molds, so that the rotatable tube is essentially full of molds. As each new mold is introduced into the rotatable tube conditioning zone, a fully cured mold is ejected from the bottom of the polymerization zone.
By this means, the number of molds within the rotatable tube remains constant, with individual molds progressing slowly through first the conditioning zone, and then the polymerization zone. This arrangement allows the polymerizable liquid within each mold to attain its equilibrium meniscus shape before entering the polymerization zone, wherein polymerization may be initiated. This arrangement, while allowing for the continuous curing of contact lenses, is not without its issues.
Predominant among these issues is line clearance. Contact lenses are produced with a range of differing parameters, most notably the sphere power. A typical power range for a contact lens offered for sale will be at least +4.00 diopters to −8.00 diopters, in 0.25 diopter steps, which represents 49 individual designs, or stock keeping units (SKU). In order to switch production from one SKU to a second SKU, it is necessary to clear all partially and fully polymerized product from the rotatable tube, since to change SKU it is necessary to either change the mold design and/or the rotational speed of the rotatable tube.
Typically this line clearance is achieved by adding mold blanks (for instance empty molds, or cylindrical plugs) into the top of the rotatable tube in place of the dosed molds, and continuing the spinning process until all the product is ejected from the polymerization zone. Once the required changes to effect the change of SKU have been completed, dosed molds can again be added one by one into the rotatable tube, with the mold blanks being ejected from the bottom of the polymerization zone until all the blanks have been cleared. This line clearance naturally can take some time, and essentially represents a period of reduced productivity.
The problems of line clearance are compounded when toric lenses are manufactured. Tonic lenses are used to correct those who have an optical defect called astigmatism. Astigmatism causes blurred vision due to the inability of the optics of the eye to focus a point object into a sharp focused image on the retina. This may be due to an irregular or toric curvature of the cornea or lens. With a toric lens, a typical power range would be with sphere powers over the range +4.00 diopters to −8.00 diopters, in 0.25 diopter steps, with at least 1 cylinder power offered in at least 6 axes, representing 294 individual SKU's.
Line clearance presents further problems if a temporary line stoppage is necessary. Should a manufacturing parameter deviation create a temporary line interruption, the full line must be cleared prior to trouble shooting or restarting. The very nature of a continuous flow system dictates that molds can only be ejected or reintroduced at a standard part rate. The larger or longer the line, the longer clearance time will be required.
The problems of line clearance can be removed if the spinning process is run as a batch or semi batch process. In this process, the rotatable tube is initially filled with dosed molds in one operation. The rotatable tube is then rotated at the desired rotation speed in order to allow the polymerization mixture contained within each mold to attain its equilibrium shape. Then polymerization is initiated by exposure to a preferred means of radiation. Ultraviolet polymerization is strongly preferred in batch processing as it allows almost instantaneous switching from zero exposure to full exposure, whereas a thermal initiation would require both heat-up and cool-down periods. The overall lens production cycle in a batch spin casting process will therefore require less time, and, consequently, be more efficient when using ultraviolet polymerization.
However, in order to utilize ultraviolet initiation in spin casting, the rotatable tubes are limited to being constructed from a material transparent to the passage of ultraviolet light. Further, the material used in the construction of the rotatable tubes must not be subject to the deleterious effects of prolonged ultraviolet exposure which may cause, for example, discoloration or mechanical degradation. For this reason, most rotatable tubes are made from glass.
While glass is an efficient material for use in spin casting in terms of UV transmissibility, the spin tube must also be able to both present an accurate and straight inner bore for the molds and must spin around its own vertical axis with minimal run out of polymerizable liquid and minimal vibration within the system. To achieve these objectives utilizing a glass rotatable tube is not without its challenges. Firstly, glass is not conducive to accurate machining. In order to accurately form the inner bore of the rotatable tube, a hot blank glass rod must be drawn onto a metal former. See, e.g., Method of Forming Precision Bore Glass Tubing, U.S. Pat. No. 2,458,934 to Everett Samuel James (filed Nov. 22, 1941) (issued Jan. 11, 1949). This process is tedious and time consuming, and may produce a tube having an inner bore that contains flaws or is otherwise imprecise.
Secondly, the glass rotatable tube must be mounted accurately into bearings at the top and bottom of the tube. Typically this is achieved by grinding a taper onto either end of the tube. Once the tube has been provided with tapers, the tube may be mounted into the bearings. The bearings must also be provided with a means for adjusting the rotatable tube so that the axis of rotation is exactly along the centerline of the inner bore (i.e. to eliminate “run-out”).
Further, since glass is susceptible to brittle failure, it cannot be exposed to high tensile stresses such that the bearing mountings should not exert undue compressive force, or any excess shear forces while adjusting run-out. This precludes the use of pre-loaded high-speed bearings and typically necessitates frequent tube alignment adjustments during manufacturing.
Still further, glass tubes are susceptible to variational influences and may exhibit some lack of continuity from the top to the bottom of the tube during the spinning process. A certain amount of transient flexure may adversely affect the accuracy of individual lenses being spun within the tube. Potential inhomogeneity within the glass itself may also contribute to varying and disparate amounts of ultraviolet light reaching the mold parts within the tube. If this were to occur through the vertical axis of the tube, certain mold parts within the tube may receive a variable level of UV radiation with possible deleterious effects.
Finally, when utilizing the prior art glass tubes in a spin casting system, the glass tubes are subject to undesirable vibrations. These vibrations in the glass tube are due to the inability to maintain a sufficiently rigid connection between the glass tube and the bearing mountings. Vibrations within a system utilizing a glass tube may generate a product that lacks sufficient precision (e.g. a contact lens with undesirable imperfections or defects).
What is needed is an apparatus, a system, and a method of mass-producing contact lenses via spin casting that overcomes the above-mentioned failings of prior art systems.